In the prior art, a vehicle-mounted video recording apparatus has been proposed, generally known as a drive recorder, that records image information captured by an onboard video camera in a continuously circulating manner (hereinafter describe as “in an endless manner”) and, in the event of an accident such as a vehicle collision, saves the thus recorded image information on another recording medium so that the images recorded before and after the accident can be used to investigate the cause of the accident (patent document 1).
As shown in FIG. 1, the drive recorder 2 is mounted inside the vehicle 1 and connected to the video camera 3 which captures the view ahead of the vehicle 1. The image information being captured by the video camera 3 is recorded in an endless manner on a semiconductor memory device incorporated in the drive recorder 2. When vehicle 1 is involved in a vehicle accident or the like, the impact is detected by a gravitational acceleration sensor 4, whereupon the drive recorder 2 determines the situation as being the occurrence of an accident and transfers the captured image information recorded on the semiconductor memory device to a recording card 5 for saving. Images recording the view ahead of the vehicle 1 for a period of several seconds before and after the accident are thus saved on the card.
The captured image information saved on the recording card 5 can be reproduced in the form of video on a reproduction apparatus 5a such as a PC to investigate the cause of the accident. In particular, in the case of an accident at an intersection where traffic lights for vehicles (hereinafter referred to as “traffic lights,” which include not only those for vehicles but also those for pedestrians and bicycles) are installed, the illumination state of each traffic light at the time of the occurrence of the accident may be captured in the recorded images, in which case the state of the traffic light provides an extremely effective means for judging the condition of the traffic at the time and the responsibility of the persons involved.
In Japan, LEDs (Light-Emitting Diodes) have been used for traffic lights since 1994. Since the power used to operate such LED traffic lights is produced by full-wave rectifying the commercial power, the applied voltage varies at twice the frequency of the commercial power, i.e., at 100 Hz in eastern Japan and at 120 Hz in western Japan. Furthermore, LEDs need a voltage greater than a predetermined level to operate. LED traffic lights are therefore designed to illuminate when a voltage greater than about one half of the supply voltage is applied. Accordingly, unlike traditional bulb-based traffic lights which continuously illuminate, LED traffic lights illuminate by flashing on and off at twice the frequency of the commercial power.
In Japan, the frequency of the commercial power is 50 Hz in the eastern half of Japan and 60 Hz in the western half of Japan, the boundary between them being generally defined by a line joining the Fuji River in Shizuoka Prefecture and the Itoi River in Niigata Prefecture. In practice, the standard power frequency is determined for each electric power company in accordance with the provisions of the power supply contract, and the frequency of 50 Hz is used in Yamanashi Prefecture and Niigata Prefecture (except some regions), while the frequency of 60 Hz is used in Nagano prefecture (except for some regions) (see FIG. 2).
The video camera 3 which transmits the video signal to the drive recorder 2 captures an image at the video signal frequency (59.94 Hz) defined by the NTSC (National Television System Committee) standard or at the video signal frequency (50 Hz) defined by the PAL (Phase Alternating Line) standard, and outputs the video signal corresponding to the captured image. Generally, TV monitors and LCD monitors are designed to display images at the video signal frequency defined by the NTSC or PAL standard.
In these circumstances, when an image of an LED-based traffic signal is captured by the video camera, there can occur cases where the color of the traffic light in the traffic signal is unrecognizable in the captured image because of the relationship between the flashing frequency of the traffic light (120 Hz or 100 Hz) and the video capture frequency of the video camera (59.94 Hz or 50 Hz).
Furthermore, images captured by the drive recorder are checked on the screen of a personal computer (PC) by reading a recording medium or the like provided in the drive recorder into the PC. However, in the case of a method that checks the captured image at a later time at some other location, the possibility of the captured images being tampered with cannot be ruled out.
If the images captured from the scene of an accident can be checked on the spot, the problem of such image tampering can be avoided. That is, it is extremely important that the images captured by the drive recorder be checked on the display of a navigation system or the like mounted in the vehicle.
However, in the prior art, when the images captured by the drive recorder are displayed on the onboard display unit, there arises the earlier described problem due to the LED traffic light flashing and the video capture timing. The reason for this will be described below.
FIG. 3 is a diagram showing the relationship between the LED traffic light flashing and the video capture timing.
FIG. 3(a) shows the case where images of the traffic light flashing on and off at 100 Hz are captured with the capture timing of 50 Hz. In the figure, “A” indicates the timing when the images are captured when the light is the brightest, and “B” indicates the timing when the images are captured when the light is the darkest. That is, in the case of “A”, the color of the traffic light is recognizable in all the captured images, but in the case of “B”, the color of the traffic light is recognizable in none of the captured images. That is, when the flashing frequency of the traffic light coincides with an integral multiple of the video capture frequency, there occur cases, as indicated by “B”, in which the color of the traffic light is unrecognizable in the captured video.
FIG. 3(b) shows the case where images of the traffic light flashing on and off at 100 Hz are captured with the timing of 45 Hz. C1 to C16 indicate the video capture timing instants. In this case, since the flashing frequency of the traffic light does not coincide with an integral multiple of the video capture frequency, the traffic light in the captured video flashes on and off at a frequency fb (Hz). For example, in the images captured at C1, C2, C5, C6, C9, C10, C11, C14, and C15, the color of the traffic light is recognizable. In this way, when the flashing frequency of the traffic light does not coincide with an integral multiple of the video capture frequency, the color of the traffic light can be recognized at some of the video capture instants.
The relationship between the flashing frequency fs (Hz) of the traffic light and the flashing frequency fb (Hz) of the traffic light in the captured video is defined by the following equation (1), wherein fr (Hz) is the video capture frequency, and n is an integer that makes fr×n a value closest to fs.fb=|fs−fr×n|  (1)
For example, when equation (1) is applied to the case of FIG. 3(b), fb=(100−45×2)=10 (Hz), which means that the traffic light in the captured video flashes on and off at 0.1-second intervals.
FIG. 4(a) is a diagram showing the flashing cycle (in seconds) of an LED traffic light in the captured video when the traffic light is captured using the NTSC or PAL standard in eastern Japan, for comparison with that in western Japan. When the traffic light is captured using the PAL standard in eastern Japan, the flashing cycle of the traffic light is 0.00 second. That is, in this case only, the traffic light does not flash. FIG. 4(b) is a diagram showing the relationship between the flashing cycle of the traffic light and the capture timing when the traffic light is captured using the NTSC or PAL standard in eastern Japan, for comparison with that in western Japan. When the traffic light is captured using the PAL standard in eastern Japan, since the traffic light does not flash in the captured video the flashing cycle of the traffic light coincides with the capture timing. This means that when the traffic light is captured using the PAL standard in eastern Japan, there can occur cases where the traffic light appears to remain off in the captured video, and in such cases, the color of the traffic light is unrecognizable in the captured video.
FIG. 5 is a diagram showing the case where an LED traffic light flashing on and off at 120 Hz in western Japan is captured at the video signal frequency (59.94 Hz) defined by the NTSC standard. In the figure, the ordinate represents the voltage applied to the LED traffic light, and the abscissa represents the time (in seconds). It is assumed here that when a voltage not lower than a threshold voltage S corresponding to one half of the supply voltage is applied, the LED illuminates and the color of the traffic light is recognizable but, when a voltage lower than the threshold voltage S is applied, the LED does not illuminate sufficiently and the color of the traffic light becomes unrecognizable. Further, curve C is a plot, as a function of time, of the voltage applied to the LED traffic light at the video capture timing.
In the case of FIG. 5, the flashing frequency of the traffic light in the captured video is 0.12 (Hz) from equation (1), and the flashing cycle is about 8.33 seconds. This means that the curve C cycles on and off in about every 8.33 seconds. In FIG. 5, during the period from time t0 to time t1 and the period from time t2 to time t3, since the applied voltage is not lower than the threshold voltage S, the LED illuminates and the color of the traffic light is recognizable. On the other hand, during the period from time t1 to time 2 in FIG. 5, since the applied voltage is lower than the threshold voltage S, the LED does not illuminate sufficiently and the color of the traffic light becomes unrecognizable. That is, throughout the period from time t1 to time 2, which lasts about 2.78 seconds (about one third of the flashing cycle), the color of the traffic light cannot be recognized. In particular, if that period overlaps the ON period of the yellow light (about two seconds), the illumination state of the yellow light becomes unrecognized. In this case, if the video captured, for example, when the vehicle entered an intersection is reproduced, it is not possible to determine whether the traffic light was showing yellow or red at that moment, and thus it is not possible to investigate the situation that occurred at the intersection.
FIG. 6(a) is a diagram showing the length of time (in seconds) during which the LED traffic light appears to remain off in the captured video. Each numeric value shown here corresponds to one third of the flashing cycle shown in FIG. 4(a). FIG. 6(b) is a diagram showing the relationship between the yellow light ON period (about two seconds) and the period during which the LED traffic light appears to remain off in the captured video. When the traffic light is captured using the NTSC standard in western Japan, the period during which the LED traffic light appears to remain off in the captured video is longer than the yellow light ON period. That is, there can occur cases where only images of the yellow light in the OFF state are captured, and in such cases, the state of the traffic light cannot be recognized in the captured video.
The above description has been given by an example in which, of the LED traffic lights, the yellow light having the shortest ON period becomes unrecognizable. On the other hand, in the case of traffic lights for pedestrians or bicycles that do not have yellow lights, the light having the shortest ON period becomes unrecognizable. That is, in the case of traffic lights for pedestrians, the flashing green light period is the shortest (about 0.5 to 1 second).
In this way, when images of an LED traffic light are captured by the video camera of the drive recorder, there can occur cases where the color of the traffic light is not recognizable in the captured images, depending on the relationship between the flashing frequency of the traffic light and the video capture timing. This problem occurs in western Japan (120 Hz) when the video camera is based on the NTSC standard (59.94 Hz) and in eastern Japan (100 Hz) when the video camera is based on the PAL standard (50 Hz).
To address this problem, it has been proposed to adjust the capture frame rate and the camera shutter speed (nonpatent document 1).
The capture frame rate of the drive recorder is adjusted so that the frame rate does not become equal to a submultiple of the frequency of the commercial power. For example, if it is set to 30.5 frames per second, blinking or flicker may appear in the captured image of the LED traffic light, but the light does not totally black out, and the illumination state of the traffic light can be recognized. The shutter speed is adjusted so that it becomes slower than the reciprocal of the flashing frequency of the LED device. For example, when the flashing frequency of the LED is 100 Hz, the shutter speed is set slower than 1/100 seconds, and when it is 120 Hz, the shutter speed is set slower than 1/120 seconds; by so adjusting the shutter speed, the illumination state of the traffic light becomes recognizable. However, adjusting the shutter speed may lead to shaky images in the case of daytime shooting, making it difficult to investigate an accident or the like by using the recorded images.
Patent document 1: Japanese Unexamined Patent Publication No. S63-16785
Nonpatent document 1: “Effects of LED Traffic Lights on Images Captured by Drive Recorder,” Japan Automobile Research Journal, Vol. 28, No. 7 (July 2006)